I would not even think of using 25X25 RHS box section to build a spaceframe chassis.
The weight gain would be unaceptable
Although I am not a race engineer, I can also forsee structural reasons as to why this material is not suitable.
If you really do not want to buy a MIG welder, why not tack your chassis with the stick welder, then hire or try and borrow a MIG to fully weld the chassis.
This will probably work out cheaper than the extra cost of RHS box.
